Wish I could help to show you how to do it. I have 3 programs to choose from here on my Mac that can all resize in one or two steps. First I adjust the image size, then save for the web by changing the save as (quality/compression) options to either 50%, medium, or web compression, depending on the program.
I've looked into various scripts for the forum so that people to put any file into the forum and have it do the work for them but they all seem even more complicated for users than it is now. I know Dave's has it built in, but he has a whole gallery system in place so it makes some sense that way.
Sorry to make it so miserable for you.